Fractures of long bones are a prevalent problem, accounting for 10% of non-fatal injuries in the USA (Kanakaris 2007). Of these, the most common are fractures of the tibial shaft, approximated to result in 77,000 hospitalisations a year in the USA (Schmidt et al 2003). The epidemiology and aetiology of tibial shaft fractures indicates a relation with risk behaviour. This type of fracture appears to be most prevalent in young men (Grutter 2000). A study by Court-Brown, 1995 found the mean age of patients with tibial shaft fractures to be 37 years, with the highest incidence occurring amongst teenage males. The two most common causes being; sports related injuries and road traffic accidents. There are several classifications described for fractures of the tibia, perhaps the most widely accepted of long bone fracture classifications in the AO/OTA classification (Arbeitsgemeinschaft für Osteosynthesefragen/Orthopaedic Trauma Association). This classification system looks solely at the pattern of fracture, not taking into consideration the local soft tissue damage (FIG. 1). Associated soft tissue injury may be classified according to the Tscherne and Gotzen classification (Schmidt et al 2003) for closed tibial fractures, and according to the Gustilo Anderson classification (Gustilo & Anderson 1976) for open fractures.
For an in-vitro biomechanical study of an instrumented nail, used for strain telemetry, the most useful of these classifications is the AO classification. This is an alphanumeric classification system for all long bone fractures. An example of a fracture classified in this way is 42-C2. “4” represents the tibia, whilst the “2” tells us this is a fracture of the diaphysis. Having described the location, the letters A, B or C are assigned to indicate the fracture type and increasing complexity. Subgroups of these, in increasing severity, are assigned by the addition of the numbers 1, 2 or 3 (Grutter 2000). Further subdivisions of these groups may be made, to indicate the number of fragments.
Of the various fracture, 42-A3 appears to be the most common, accounting for 23.9% of tibial diaphyseal fractures (Court-Brown 1995).
Treatment of these fractures is broadly divided into two categories, conservative and surgical. Conservative therapy involved the use of a plaster-cast or functional bracing. Surgical treatment can involve either open-reduction and internal fixation (ORIF) of intramedullary (IM) nailing. A META-analysis comparing conservative treatment to ORIF found that despite significantly decreased risk of superficial wound infection, casting resulted in a lower rate of union at 20 weeks (p=0.008) (Littenburg et al. 1998). Additionally casting is limited by the severity of the fracture and deformity, with initial moderate or severe displacement increasing the rate of delayed of non-union from 9% to as much as 27% (Schmidt et al 2003).
IM nailing appears to be the preferred method of treatment for the majority of tibial fractures (Schmidt et al 2003). This suggestion is supported by a Randomised Control Trial (RCT) which shows IM nailing to result in faster union and a decrease in the rates of malunion, in comparison to conservative treatment (Hooper G J 1991).
Delayed or non-union are a major concern with tibial fractures. On a “best case scenario” calculation the cost of one tibial non-union is estimated to be £16,330, with 20% being direct costs of treatment and 80% due to indirect costs (Kanakaris 2007). The reported incidence of delayed union shows a great degree of variability due to the arbitrary definitions used. Generally delayed union of the tibia is recognised at 20 weeks, however, earlier detection may be possible. One could think of delayed union as the point at which altering the treatment may be considered, in order to achieve union (Phieffer & Goulet 2006). The definition of non-union is broadly accepted as the presence of no radiographic evidence of healing for three consecutive months, in a fracture of at least 9 months of age. The prevalence of delayed and non-union is reported to be 4.4% and 2.5% respectively. However, in open fractures, delayed union may be as high as 41%, requiring further treatment before union is achieved (Phieffer & Goulet 2006).
Treatment for delayed union varies in light of the cause. This can, broadly speaking, involve stabilisation, re-nailing, bone-grafts, adjunct therapy such as electrical stimulation, ultrasound or biological adjuncts such as Bone Morphogenic Protein (BMP). However timing is key to success as early diagnosis and treatment of delayed union can save the patient from considerable periods of disability and pain (Phieffer & Goulet 2006), whilst also having an impact on health economics due to a reduction in morbidity.
Various methods have been used to ascertain the end point of healing of fractures. This is fundamental knowledge to clinicians so as to advise patients on appropriate load bearing in the injured limb or to diagnose the risk of delayed or non-unions.
Currently there is a lack of a gold standard method which supplies sensitive data, good repeatability as well as ease of use. Serial radiographs and manual manipulation, often used in conjunction, are subjective and show inter-clinician variability. The inaccuracy and complexity of using dexa-scans, vibrational analysis, scintigraphy and ultrasound has also eliminated them as potential measurement tools.
Telemetry
An IM nail acts to provide stability, whilst transmitting rotational, bending and compressive forces across the fracture site and maintaining anatomical alignment of the bone. The IM nail also acts as a load sharing device, gradually shifting the load to the bone, as it heals.
Telemetry enables the direct measurement of strain and load carried by an appropriately instrumented IM and hence gives an indication of the progress of fracture repair without disrupting fracture healing. An example of a telemetric orthopaedic system is disclosed in WO 2007/025191, which is herein incorporated in its entirety. In addition to its clinical use, such methodology proves to be of great benefit toward increasing our understanding of fracture healing and its biomechanics. It allows optimisation of post-operative patient care, assessing the role of different activities on skeletal loads to identify which are most appropriate for providing the desired mechanical environment (Schneider E, 2001).
Strain gauges, which enable the direct measurement of the load applied to the nail, are conventionally located in multiple recesses in the outer wall of the nail and hence have the potential to cause changes in the biomechanical properties of the nail. This in turn could lead to local weakening or stress concentration.
We have identified redundancy associated with the provision of strain gauges at multiple locations on a nail and have identified: firstly an optimal position for a recess comprising a plurality of strain gauges and secondly an optimal orientation of the strain gauges relative to the longitudinal axis of the nail. The strain gauges are capable of monitoring the strain in a nail when it experiences either off-set axial compression, torsional forces or three/four point bending forces.
The identification of the optimal positioning and orientation of the strain gauges will facilitate the generation of a single commercial design of an IM nail which can be used with varying fracture patterns.
Radiostereometric Analysis (RSA)
In vivo measurement of three-dimensional (3D) displacement of prosthetics or body parts was pioneered by Gam Selvik in 1974 (Bragdon et al 2002). RSA is also referred to as radiostereometry or roentgen stereophotogrammic analysis.
RSA measurements can be obtained using pairs of simultaneous radiographs taken repeatedly over time. Tantalum bead markers are implanted into the body part or implant segment under study with at least three non-colinear beads needed to define each rigid body subject to scrutiny (Valstar et al. 2005). A 3D coordinate system is achieved by way of a calibration cage embedded with tantalum beads in well defined, immoveable positions. Two radiographs placed side-by-side, in a uniplanar arrangement or at a 90 degree angle to each other, in the case of a bi-planar arrangement (Valstar et al. 2005) are used to establish the 3D coordinates of the markers, and displacement between the rigid bodies can be calculated (Madanat et al. 2006) using commercially available RSA software systems.
Whilst RSA is a “gold standard” technique for assessing fixation and migration of joint replacements and determining micromotion of the bone, this technique has not be considered for measuring inter-fragmentary movement in long bone fractures fixated with an orthopaedic fixation device.
We have identified that RSA can be used accurately and precisely to measure inter-fragmentary movement in a long bone, such as a tibia, fixated with an IM nail before and after reduction of the fracture.